Mannich emulsions provide the lowest cost cationic polyacrylamide emulsion polymer suitable for sludge dewatering. The emulsions are prepared from a nonionic polyacrylamide polymer which is made cationic by performing a Mannich reaction on the amide groups along the polyacrylamide backbone. The Mannich reaction comprises reacting the amide groups with both a dialkylamine and a formaldehyde source to form a mixture of tertiary amino structures and water. The tertiary amines may then be quaternized in a conventional manner. The general Mannich reaction may be performed in a number of different ways including (i) adding the formaldehyde and amine separately to the polyacrylamide polymer, (ii) prereacting the amine and formaldehyde to form an adduct which is then added to the polymer and (iii) in a continuous process which involves adding a mixture of the individual reactants at various points along a continuously moving liquid stream. Numerous variations of these processes are disclosed and well known in the art.
The water-in-oil Mannich emulsions produced by these processes which are sold into the municipal market have three significant drawbacks relating to storage and handling. First, the emulsions lose their mechanical strength, i.e. loss of flow properties leading to stringiness and complete gelation of the emulsion. Second, the emulsions lose their chemical stability, i.e. a loss of cationicity, and a reversal of the Mannich reaction leads to the loss of flocculating performance. Third, the polymers crosslink to such an extent that they become water-insoluble.
Various stabilizers have been suggested in the prior art as solutions to overcome the stability problems. For example, U.S. Pat No. 4,179,370 discloses the use of amino salts and amines and U.S. Pat. No. 4,010,131 discloses halogen-free oxygen-containing inorganic acids such as sulfurous acid. None of these stabilizers, however, have been found to sufficiently stabilize the Mannich water-in-oil emulsions of this invention and thus research for improved stabilizers has continued.
The problem of stabilizing Mannich products has also existed with dilute aqueous solutions of acrylamide polymers. For instance, U.S. Pat. No. 3,988,277 discloses the general use of formaldehyde scavengers to provide stabilization to about 4% polyacrylamide polymer aqueous solutions. Specifically, the patent discloses the use of any one of guanidine, hydrazine, ammonia, morpholine, dimethylamine, and urea, with the most effective of these being hydrazine. As shown by comparative example below, when used with the water-in-oil emulsions of this invention as opposed to dilute solutions hydrazine provided no stabilizing effect but rather it de-stabilized the products. Similarly, ammonia in the form of ammonium chloride and urea had little or no effect on the stability.
Accordingly, it is the object of the present invention to improve the chemical stability of water-in-oil emulsions containing Mannich reaction products in high concentrations.